Transistors typically include at least one gate over a channel region, and source and drain regions on opposite sides of a channel region, and at least one gate that regulates current flow through the channel. Contacts to the source and drain regions are often made by first forming a metal silicide on the base source drain material, then depositing a dielectric over the transistor, patterning contact trenches in the dielectric, and lastly forming metal source drain contacts in the contact trenches.
The contact resistance between the metal contacts and the source drain material is an important consideration. Dopant implantation can be used to reduce the contact resistance. Boron is common dopant for p-type transistors. Boron, however, has a low solubility in materials with a high Ge content. For example, boron has a solubility of about 5.5×1018 at./cm3 in pure germanium. A low solubility results in a higher contact resistance. Thus, boron is not an ideal dopant for Ge-based transistors.
Accordingly, improved techniques for source and drain contact formation in Ge-based transistors having reduced contact resistance would be desirable.